Relationship between total body clearance of caffeine and urine flow rate in elderly men

The total body clearance (CL), renal clearance (CLR), and nonrenal clearance (CLNR) of caffeine from plasma were determined following the intravenous administration of caffeine (4 mg kg-1) to ten healthy men (aged 66-86 years) on three separate occasions. Positive correlations were observed between CL and urine flow rate (UFR), between CLR and UFR, and between CLNR and UFR (r = 0.8947, p = 0.0002; r = 0.8832, p = 0.0003; and r = 0.8920, p = 0.0002, respectively). Previous studies have established similar relationships between CLR and UFR for caffeine and its initial dimethylxanthine metabolites; theophylline, theobromine, and paraxanthine. A relationship between CL and UFR has not been reported previously.